There's really no reason to have this anymore. It was (AFAIK) introduced years ago
as a performance-saving measure, then was moved to Advanced Preferences in 1.13 since
modern PCs mostly have no issues. With accelerated rendering, there's even less of a
reason to have this.
Even though we switched to encrypted local password storage a while back, any passwords
saved with the old plaintext method remained in player's preferences file and were loaded
by the game where there's no reason they couldn't have been directly accessed.
A few of them are quite small, so this has no performance effect on those, but a few others
are potentially quite large, such as the one in campaign_server.cpp.
This fixes#2156.
preferences::add_acquaintance replaces the existing friend entry if the given username
has already been added to the acquaintance list, but the prefs dialog always added a
new row unconditionally. I've changed it to only add a new row if the username creates
a new entry, else replace the appropriate exiting row.
The game now supports @ or = in usernames for the purpose of saving them to a file.
Though I don't think these are allowed currently, it's probably better to
support it from the start rather than having things break if they later
become allowed.
